Fluorescence of dyes in response to changes in membrane potential has been measured in squid axon. It is not, however, obvious that the same dyes could be used as molecular probes to follow membrane potential changes in cardiac muscle. In fact, preliminary data in this laboratory indicates that such assumptions are unfounded. We have, therefore, chosen to systematically establish the relation between the structure of a particular dye and the physiological function to be measured. From physico-chemical structural considerations we shall select dyes that will bind to the myocardial membrane and whose fluorescence changes will reflect the changes in membrane potential. This type of approach will be extended to monitor some of the steps in the excitation-contraction coupling processes, e.g., the control of contraction by the action potential, "triggering" role of calcium current on internal activator Ca ion 2, and the time course of release and uptake of Ca ion 2 by internal membrane systems, etc. The fluorescence measurements will be combined with the voltage clamp technique (which is already operational in our laboratory) to determine the temporal relation of fluorescence activity with membrane currents, membrane potential or membrane conductance changes. As we gain expertise in the properties of dyes and their interaction with different subcellular fractions it is hoped that more than one dye could be excited at the same time to monitor simultaneous cellular functions such as membrane potential and release or uptake of Ca ion 2, etc. The application of dyes to the myocardium if successful as physiological probes would not only provide a great tool for basic science research but also be of great help in clinical diagnosis of diseased state.